Document the storage interfaces from the user perspective

[francesshei]

The label method docstring currently reads:

        Should label the cells with given labels.

        :param cells: Array of cells in this set to label.
        :type cells: numpy.ndarray
        :param labels: List of labels
        :type labels: list[str]

Could be useful to explicitly state that the function assigns each label in the list to each cell.

[Helveg]

This docstring is for the interface of PlacementSet, aimed at developers trying to implement their own PlacementSet for a new storage engine. In that case, they should know that this function should label the cells with given labels.

That said, encountering this docstring in the BSB docs as a user is quite confusing. Instead of linking to the bsb.storage.interfaces.PlacementSet, we should link to bsb_hdf5.PlacementSet as the de facto default implementation.